Method of decolorizing paper mill effluent liquid

ABSTRACT

Effluent liquid normally discharged by paper mills is ecologically undesirable for return to streams, lakes or the like by reason of the presence of color-imparting substances, e.g., color bodies, desirably removed by a process which includes mixing therewith an excess of calcium carbonate, followed by a combination at a pressure above atmospheric with a fluid rich in carbon dioxide, serving to convert the calcium carbonate to soluble calcium bicarbonate which is in turn discharged to atmospheric pressure whereby the calcium bicarbonate reconverts to calcium carbonate precipitate, said precipitate attracting and sweeping said color-imparting color bodies; the precipitate and color bodies being readily separable from the color-free effluent which may be returned to the source, stream, river or lake or reintroduced as fresh water into the pulping process as needed.

United States Patent [191 Rolfe [451 Sept. 3, 1974 METHOD OFDECOLORIZING PAPER MILL EFFLUENT LIQUID [75] Inventor: Ormond E. Rolfe,Valdosta, Ga.

[73] Assignee: Owens-Illinois, Inc., Toledo, Ohio 22 Filed: Feb. 16,1973 [21] Appl. No.: 333,285

[52] US. Cl 162/29, 162/33, 162/189,

Primary Examiner-S. Leon Bashore Assistant Examiner--Alfred D. Andrea,Jr.

Attorney, Agent, or Firml-Ienry P. Stevens; Edward J. Holler; Paul F.Stutz [5 7 ABSTRACT Effluent liquid normally discharged by paper millsis ecologically undesirable for return to streams, lakes or the like byreason of the presence of color-imparting substances, e.g., colorbodies, desirably removed by a process which includes mixing therewithan excess of calcium carbonate, followed by a combination at a pressureabove atmospheric with a fluid rich in carbon dioxide, serving toconvert the calcium carbonate to soluble calcium bicarbonate which is inturn discharged to atmospheric pressure whereby the calcium bicarbonatereconverts to calcium carbonate precipitate, said precipitate attractingand sweeping said color-imparting color bodies; the precipitate andcolor bodies being readily separable from the color-free effluent whichmay be returned to the source, stream, river or lake or reintroduced asfresh water into the 1 pulping process as needed.

7 Claims, 2 Drawing Figures 210/45, 210/51, 423/159, 423/165 [51] Int.Cl. D21c 11/00, D21f1/82 [58] Field of Search 162/29, 33, 38, 39, 190,162/189; 210/43, 45, 51, 47; 423/159, 165, 427, 430

[56] I References Cited UNITED STATES PATENTS 2,462,277 2/1949 Naugle423/165 2,801,264 7/1957 Thomsen 162/38 X 3,216,788 1l/l965 Hoeft423/159 3,262,877 7/1966 Compte, Jr... 162/190 X 1 3,639,206 2/1972Spruill 162/29 3,736,254 5/1973 Croom 210/51 MUD C0.CO5

V KRAFF MILL MlX PRESSURE V SSEL. CLARlW 25 g MUD 2'9 STABILIZED LIQUIDPATENIEDSEP 31w MEI 1 0f 2' MOmmwwEZOU METHOD OF DECOLORIZING PAPER MILLEFF LUENT LIQUID The present invention relates broadly to papermanufacture. More particularly, the present invention relates to pulpingand, even more specifically, to the pulping of wood by the so-calledkraft pulping system employing sodium hydroxide and sodium sulfide asthe active pulping chemicals. Generally, of course, in the pulpingindustry, timber, that is, cut timber or trees, is cut and converted tochips which are thereafter digested in a paper mill either by physical,semi-chemical or totally chemical techniques to form a pulp or slurry ofthe cellulose fibers which are separated from the liquor and collectedby means of a Fourdinier machine, following which water is extracted andthe formed sheet of fibers is collected, dried and rolled into sheetstock. Pulping or digesting can be accomplished by a variety of chemicalsubstances such as sodium sulfate, sodium sulfite, sodium sulfide,alkali metal bisulfites, hydroxides, etc.; which chemicals are employedas cooking liquors to attack the lignins and other binding constituentswhich hold the cellulose fibers together in the form of the wood as itgrows and matures. Over the years, pulping economy has made necessarythe employment of a variety of chemical recovery operations. Thesechemicals, for example, sodium chemicals, can be treated after pulpingand essentially recovered, revitalized by treatment and addition ofmakeup chemicals to yield active liquors available for reintroduction asfresh pulping chemicals to effect digesting separation of the desiredcellulose fibers.

Pulping and the various chemical recovery techniques require tremendousamounts of water, usually taken from rivers, streams, lakes and thelike. By reason of the contact of the water with the wood, with pulpingchemicals, with the various by-products of the chemical reactions takingplace and the pickup in the water of the waste fiber bindingconstituents such as lignins, organic by-products, etc., the waterbecomes very contaminated and considerably discolored.

Water which is discolored is, of course, very undesirable andunacceptable as a replacement for water drawn from natural supplies. Inview of the tremendous amounts of water employed in industrial papermill and similar operations, the problem of discoloration is ofconsiderable importance which takes the form of a stimulus to remove orreduce contamination and discoloration.

lt has been determined that the coloration, which is in part aby-product of pulping, is imparted to the water by what are referred toin the art as color bodies. These color bodies include chemical entitiessuch as lignins, tannins and long chain organic polymers of resinouscharacter which are by-products of the chemical attack of the variouspulping chemicals employed on the above referred to bonding cementswhich hold the cellulose fibers which make up the wood or timbertogether.

, It has also been theorized that the color bodies are composed of areaction product of calcium ion with either phenolic or enolic groupswhich are by-products of the hydrolysis or lignin. The color bodies orcomplexes, if you will, are extremely small, measuring in the colloidalrange, which makes the removal of these color bodies quite difficult.

There are known in the art a number of techniques for attempting to dealwith the problem of discoloration, impurities and contaminants in papermill effiuent. Thus, in U.S. Pat. No. 3,120,464, there is disclosed amethod wherein spent liquor, containing organic color bodies, is treatedwith lime, that is, calcium oxide, serving to absorb the color bodiesfor separation from the spent liquor. The so-called lime technique hasbeen refined by the expedient of employing massive dosages of limewhich, unfortunately, adds considerably to the expense. Reducing theamount of lime to reduce the attendant expense results in the formationof a gelatinous precipitate which is difficult to separate by filteringor like separation operations. The gelatinous precipitate is believed tobe a complex reaction product of lime with the organic constituent ofthe binding cement holding the fibers together.

U.S. Pat. No. 3,531,370 also discloses a process for removing colorbodies by the use of lime and additionally a cellulosic fiber to obtaineffective settling.

U.S. application Ser. No. 149,534, filed June 3, 1971, entitled SystemFor Removing Color From Paper Mill Liquid Waste now U.S. Pat. No.3,736,254 issued May 29, 1973, and U.S. application Ser. No. 301,132,filed Oct. 26, 1972, entitled Method Of Decolorizing Waste ProcessLiquid Discharged By A Paper Mill," said applications being assigned tothe same assignee as the present application, are also disclosive ofprocesses for overcoming the problem of discoloration of mill effluent.

It is an object of the present invention to provide a method ofeffectively accomplishing removal of coloration or discoloration presentin paper mill liquid effluent, said effluent including both liquid fromthe pulping process and liquid from the machine room.

It is a particular object of the present invention to provide such amethod which is effectively integrated into existing pulping operationsand attendant and collateral chemical recovery operations.

It is a particular object of the present invention to provide such amethod which employs as additives readily available materials andsubstances and which result in by-products which are redirectable andare reemployable in the recovery and/or pulping system.

It is a significant object of the present invention to provide a systemwhich does not feature the utilization of the relatively expensive limeand furthermore is avoida tive of the production of any gelatinousbyproductbut rather results in a relatively easily separable precipitatewhich is easily handled and is advantageously directed to recoveryoperations already employed.

The foregoing and, as well, other objects of the present invention willbecome apparent to those skilled in the art from the following detaileddescription taken in conjunction with the annexed sheets of drawings onwhich there are presented, for purposes of illustration only, severalembodiments of the present invention including a schematicallyillustrated sequence of steps and, additionally, a somewhat diagrammaticillustration of the inventive method integrated into a kraft pulping andchemical recovery system.

1N THE DRAWINGS FlG. l is a schematic block and line flow diagramillustrating the flow of the kraft mill effluent through the sequence ofoperational steps for combination with various additives to yield thefinal substantially colorfree liquid; and

FIG. 2 is a somewhat more detailed block and line flow sheetillustrating the movement of the mill effluent as in FIG. 1 butadditionally showing the flow movement of the additives from and to thevarious streams of liquids in a typical kraft chemical recovery system.

In accordance with one embodiment of the present in-.

vention, this treatment is effected by conjointly mixing, with thediscolored effluent, lime mud and carbon dioxide at a pressure greaterthan atmospheric as will favor a chemical reaction converting thecal'cium carbonate to soluble calcium bicarbonate.

Considered in a more commercial sense, the present invention envisionsthe employment of readily available calcium carbonate combined, inexcess, with the mill effluent liquid to be decolorized,'followed by acombination, under pressure, with readily available carbon dioxide,preferably in the form of lime kiln stack gases, to convert the calciumcarbonate to soluble calcium bicarbonate, following which the pressureis released, resulting in a reversal of the pressureinduced reac-tionandconsequently the formation of insoluble calcium carbonate once again,which precipitation together with gravity effectively attracts andcarries color bodies in conglomeration therewith; the precipitate andcolor bodies being easily separated by simple clarification; theeffluent being available for reintroduction into the source, stream, oremployed for process liquid free of any coloration. The reprecipitatedcalcium carbonatev and the color bodies carried thereby can-be readilydirected to the limekiln for conversion to reusable lime.

Reference should now be had to FIG. 1 for a brief and simplifiedillustration of the sequence of steps and operationsconstituting thetreatment, in accordance with the present invention, of the discoloredaqueous kraft mill effluent proceeding from the entire kraft milloperation. As can be seen, starting at the left of FIG. 1, thediscolored mill effluent is directed together with a slurry of calciumcarbonate mud to mixing tank 11. The resulting mixture is pumped by pump13 through pressure-maintaining check valve 15 to pressure vessel 17into which is essentially simultaneously introduced, through line 19,pressurized carbon dioxide present in by-product stack gases emanatingfrom lime kiln 21; the stack gases containing the carbon dioxide beingpressurized by compressor 23. The pressure vessel is designed in termsof capacity or volume to provide a residence time, of the mixture ofKraft effluent and calcium carbonate in admixture with pressurizedcarbon dioxide, of from about 2 to 5 minutes, whereby essentially all ofthe calcium carbonate is converted at the higher pressure to watersoluble calcium bicarbonate. Pressure is maintained within the pressurevessel by a pressure relief valve 25 in the outlet line 17a of vessel17; saidvalve being predeterminedly set to maintain a desired pressuredesirably within the range of about 20 to pounds per square inch. Itwill be appreciated that the quantity of efiluent to be treated togetherwith the amount of calcium carbonate slurry and quantity of carbondioxide, bearing in mind the concentration of the carbon dioxide in thelime kiln exhaust gases, will determine the size and capacity of thepressure vessel 17. The pressure relief valve 25, in addition tomaintaining the desired pressure within the vessel, serves to releasethe liquid, e.g., aqueous calcium bicarbonate, to clarifier tank 27maintained at atmospheric pressure; the release in pressure initiatingthe formation of insoluble calcium carbonate. The calcium carbonateprecipitate increases in amount and propagates, gradually falling to thebottom of the tank, sweeping with it as it grows in size the colorbodies. The efficiency of color removal according to this method isbelieved in part due to the fact that the initial precipitate is quitesmall and serves to attract in admixture therewith the extremely small,in fact, colloidally sized, color bodies. The precipitate continues toform and grow, carrying with it differing sized color body entities. Inaccordance with a preferred embodiment, the removal of the color bodiesis very efficiently accomplished by an excess of calcium carbonate whichis first solubilized in the pressure vessel and then is reprecipitatedin the clarifier tank, providing thereby a vast multitude of formedinsoluble molecules of insoluble calcium carbonate which propagate andgrow in the manner above described, resulting in a mass sweeping actionupon the body of liquid in the clarifier tank. The precipitate falls, asindicated, to the bottom of the tank, carrying with it the color bodies;all of which are removed through line 29 as mud'and redirected tocalciumcarbonate mud storage and/or to the lime kiln for recovery ofcalcium oxide and reproduction of carbon dioxide. Simultaneously withthe initiation of precipitation of the calcium carbonate, there isinitiated a release of carbon dioxide as a gas which bubbles, as itwere, to the surface and passes harmlessly to the surroundingatmosphere.

Liquid constituting the tops in the clarifier tank 27 is passed via line31 to a secondary treatment and storage tank 33, providing additionaltime for stabilization of the liquid to a final pH ranging from about7.8 to about 8.4. Here again, any residual precipitate which may andfrequently does form is withdrawn at the bottom and directed to thecalcium carbonate while effluent water is drawn off as stabilized,essentially colorfree water adapted for reintroduction into the stream,river or lake from which originally obtained or available forreintroduction to the mill operation at whatever point fresh color-freewater is desired or needed. The stabilization of the clarified liquid intank 27 may be hastened by utilizing a violent mixing operation.

To more clearly understand and visualize the integratability of theprocess of the present invention into a commercial mill operation,reference should be had to FIG. 2 wherein the sequence of operations ofFIG. 1 l

tem is generally meant the reactivation of green liquor by the additionof lime to form white liquor which is composed of the active chemicals,sodium hydroxide and sodium sulfide. This white liquor is utilized asthe prime active chemical for the pulping operation not illustrated. Asis readily known, however, the pulping proceeds in large tank digesterswhere the timber in the form of wood chips are essentially cooked in thewhite liquor. The cellulose fibers of the wood are liberated by theattack of the active chemicals on the binding constituents such aslignins, etc., with the chemical becoming gradually depleted in activityand the liquor discolored by the by-products, ultimately resulting in amass of unrefined cellulose fibers in black liquor. The fibers areseparated, washed, continuously collected as a sheet on the Fourdiniermachine and dried into paper roll stock, while the black liquor, rich insodium values, is concentrated in a large capacity evaporator, andfinally burned to drive off organic and combustible constituents,resulting in a molten smelt which is passed into a dissolving tankcontaining water and, as Well, amounts of various wash water from otheroperations as will be described. The liquid in the dissolving tankcontaining the dissolved smelt is referred to as green liquor. It iscomposed principally of sodium carbonate and sodium sulfide and ispumped via line 43 to the causticizing system 41 where lime, calciumoxide, is introduced, as previously described, resulting in theformation of fresh white liquor for further digestion of the next batchof wood chips.

- The causticization of green liquor to white liquor involves a chemicalreaction having, as a by-product, a slurry of calcium carbonate, CaCOreferred to as mud. The mud is drawn through line 48 to a mud washer andthence to mud storage 51. It may be incidentally noted that the calciumcarbonate mud wash water is stored as weak wash in a storage tank 50from which it is directed to the smelt dissolving tanks to assist thesolubilizing of the molten smelt to produce green liquor. Calciumcarbonate mud is a plentiful byproduct of the causticization systememployed in the typical kraft mill and in fact historically presented aproblem of disposal. However, it wassoon realized that the calciumcarbonate could be converted to lime, CaO. Thus, as shown, an amount ofthe lime mud is burned in lime kiln 2'1 employing whatever fuel iseconomically available, usually natural gas; the calcium carbonate atthe temperatures maintained decomposing to ultimately form calcium oxidepowder which is conveyed via line 21a to the lime storage 46. Theburning of the fuel results, of course, in flue gases while thedecomposition of the CaCO yields carbon dioxide gas typically measuringto percent of the lime kiln stack gases. These are normally passed tothe atmosphere as they are largely ecologically harmless.

It can be appreciated from the description hereinabove that the practiceof kraft chemical recovery process inherently yields a plentiful supplyof calcium carbonate and a plentiful supply of carbon dioxide.

In accordance with the, present invention, both of these materials areemployed and their use was described in the description of FIG. 1. Byway of reiteration and referring back to FIG. 2, an amount of thecalcium carbonate mud is mixingly combined in tank 11 (appearing in thelower left of FIG. 2) with the discolored kraft mill effluent and pumpedvia pump 13, through check valve 15, into the pressure vessel 17. Si-

multaneously, stack gases emanating from the lime kiln 21, or at least aportion thereof, are compressed in compressor 23 and piped to thepressure vessel 17 for reaction with the mixed slurry components,resulting in the formation, usually 2 to 5 minutes residence time, ofsoluble calcium bicarbonate which is released via the relief valve 25into the clarifier tank 27. The harmless carbon dioxide gas is given offsimultaneously with the formation of insoluble calcium carbonateprecipitate which falls, together with attracted color bodies, to thebottom, as described, and is pumped via line 29 back to the mud storage51. The aqueous effluent from clarifier 27 passes via line 31 into thestorage vessel 33 in the manner described hereinbefore in FIG. 1 or inaccordance with a subsidiary embodiment of the present invention,violently agitated in tank 60 to hasten the stabilization and residualprecipitation, if any. A secondary clarifier tank 62 may then be used toseparate the stabilized liquid which may be returned to the process orstream while any residual precipitate with agglomerated color bodies canbe directed back to mud storage via line 62a.

From the foregoing description, it will be appreciated that readilyavailable by-product materials are utilized in treating discolored kraftmill effluent by a sequence of steps which are easily integrated intothe Kraft mill operation, thereby accomplishing effective removal of thecolor bodies causing the discoloration and at the same time regenerationof one by-product, namely, calcium carbonate, for further conversion tolime which is, as described, usable in producing digesting liquor.

It has thus been further demonstrated that the process in accordancewith the present invention is emminently practicable for a kraft milloperation since the kraft pulping process inherently yields an abundanceof calcium carbonate and carbon dioxide by-products of use in theprocess of the invention.

Contemporaneous with the foregoing, it is an advantage of the presentinvention that different levels or concentrations of discoloration ofthe mill effluent can be readilyaccommodated. For example, should it beobserved that theeffluent discoloration is or has increased to a higherlevel, the efficiency of removal of the color bodies causing thediscoloration can be increased in any one or more of the following ways.In the first place, efficiency of color body removal can be improved byincreasing the amount of soluble calcium bicarbonate since thereconversion of the calcium bicarbonate to insoluble calcium carbonateprecipitate will result in a larger amount of precipitate, serving assites for attracting the color bodies and ultimately sweeping them asthe precipitate forms and falls by gravity to the bottom of the storagevessel or clarifying tank. The increase in production of soluble calciumbicarbonate can be accomplished by an increase in the pressure withinthe pressure vessel and/or an increase in the amount of lime mudcombined with the kraft mill effluent and/or an increase in the amountof lime kiln stack gases (carbon dioxide) introduced into the mixture oflime mud and discolored kraft mill effluent. By way of example, it hasbeen determined that a kraft mill having a daily operating productionmeasuring about 850 tons of pulp will necessarily discharge about 10million gallons of effluent and, if the kraft mill referred to includesa lime kiln recovery system for production of reusable calcium oxide,will yield or produce lime mud (calcium carbonate) at a rate of about475-600 pounds per minute. The lime kiln combustion of this lime mud torecover the lime will necessarily yield stack gases, measuringapproximately 20-25 percent carbon dioxide, at a rate of about 45,000cubic feet per minute. The justdescribed values in terms of effluent,yield of lime mud and stack gases have been correlated with the practiceof the present invention, conducted with differing pressures within thepressure vessel 17, as illustrated in either of FIGS. 1 and/or 2. Table1 below contains the tabulated data,at different pressures, of dry mudand lime kiln stack gas capacity, envisioning a kraft mill effluentthroughput of 7,000 gallons per minute; which rate corresponds to the 10million gallon per day yield.

TABLE 1 Lime Kiln Stack Gases cu ft/min 20% CO Dry Mud (CaCO Pressurelbs/min As can be seen, higher rates as to lime mud and carbon dioxideare achieved at the higher pressure values which necessarily results inan increase in the production of soluble calcium bicarbonate andnecessarily in an increase in the amount of insoluble calcium carbonateas the composite liquid is passed from the pressure vessel to theclarifier tank whereby the increased amount of precipitate is capable ofefficiently attracting and sweeping therewith a greater amount andquantity of discoloration-causing, color bodies.

The practice of the present invention, in addition, of course, toremoving coloration, desirably affects the biological oxygen demand(BOD) and the chemical oxygen demand (COD). Color has been found to bereduced by as much as 95.3 percent, while the BOD has been reduced by asmuch as 50 percent and the COD has been reduced by as much as 78percent.

While there has been disclosed with considerable detail certainpreferred manners of performing this invention, it is not intended ordesired to be solely limited thereto, for the procedure may be modified,the precise proportions of the materials utilized may be varied, andother materials having equivalent properties may be employed if desiredwithout departing from the spirit and scope of the invention unless suchdepartures would offend and violate the language of the appended claims.

1 claim:

l A method of reducing color present in paper mill effluent liquid whichcomprises:

l. mixing, into the effluent liquid, a liquid includin an excess ofcalcium carbonate, CaCO,,,

2. pressurizing said liquid to greater than atmo-- spheric in thepresence of a fluid including an excess of carbon dioxide, CO wherebysaid calcium' carbonate, CaCO is substantially converted to solublecalcium bicarbonate, CaHCO and 3. discharging the pressurized liquid toatmospheric pressure whereby said calcium bicarbonate, CaH- COsubstantially converts to insoluble calcium carbonate, CaCO as aprecipitate carrying therewith color-imparting substances present insaid paper mill effluent liquid.

4. A method of reducing color present in paper mill effluent liquidwhich comprises:

1. mixing into the effluent liquid a liquid including an excess ofcalcium carbonate, CaCO;,,

2. pumping said mixture under pressure into a tank together with a fluidincluding an excess of carbon dioxide, CO and maintaining said mixturetherein under a pressure greater than atmospheric for a time sufficientto effect substantial conversion of calcium carbonate, CaCO into solublecalcium bicarbonate, Cal-[C0 3. discharging the pressurized liquidcomprising essentially calcium bicarbonate, CaHCO to atmosphericpressure whereby said calcium bicarbonate, Cal- C0 essentiallyconverts'to insoluble calcium carbonate, CaCO as a precipitate, carryingwith said precipitate, in agglomerated fashion, color-impartingsubstances and entities carried in said paper mill effluent,

4. separating effluent liquid from precipitate and 5. exposing saideffluent to a stimulus to effect residual precipitation, formation ofcolor bodies and stabilization of effluent pH to from 7.8 to 8.4.

5. In a system of pulping wood by the kraft process wherein sodiumhydroxide and sodium sulfide are the active pulping chemicals combinedwith wood chips to attack the binding components and resulting in theliberation of cellulose fibers and wherein black liquor recovered fromthe pulping reaction is concentrated and burned to recover sodium valuesin the form of green liquor including sodium carbonate and sodiumsulfide, which green liquor is thence causticized with recovered lime toyield fresh pulping chemicals and a by-product comprising calciumcarbonate mud, a portion of said calcium carbonate being desirablyburned to yield reusable lime for said causticizing and resulting inbyproduct gases rich in CO said pulping process and recovery processresulting in a considerable amount of mill effluent liquid which isnormally ecologically undesirable due to discoloration caused by thepresence of color bodies; the improvement wherein said effluent millliquid is treated in accordance with the following series of stepsintegrated into the pulping process just described, said steps being asfollows:

1. mixing with the discolored mill effluent liquid an amount ofsaidcalcium carbonate slurry liquid separated from the causticizingreaction of lime with sodium carbonate and sodium sulfide,

2. combining said mixture under a pressure greater than atmospheric withsaid CO by-product of the burning of calcium carbonate mud, therebyconverting calcium carbonate to soluble calcium bicarbonate,

3. discharging the liquid comprising calcium bicarbonate into a settlingtank at atmospheric pressure whereby said calcium bicarbonate convertsto insoluble calcium carbonate as a solid precipitate which attracts andcarries color bodies in agglomeration or admixture therewith,

3. expose the resultant of steps (1) and (2) to a reduction in pressureto atmospheric pressure to convert soluble calcium bicarbonate toinsoluble calcium carbonate which forms a precipitate of said calciumcarbonate, said precipitate attracting and carrying color bodiestherewith, and

4. separate said precipitate and color bodies from said effluent.

7. The method as claimed in claim 6, wherein said pressure measuresabout 20 to pounds per square inch gauge.

2. combine said step (1) mixture under a pressure greater thanatmospheric with carbon dioxide, CO2, thereby converting said calciumcarbonate to soluble calcium bicarbonate,
 2. combining said mixtureunder a pressure greater than atmospheric with said CO2 by-product ofthe burning of calcium carbonate mud, thereby converting calciumcarbonate to soluble calcium bicarbonate,
 2. pumping said mixture underpressure into a tank together with a fluid including an excess of carbondioxide, CO2, and maintaining said mixture therein under a pressuregreater than atmospheric for a time sufficient to effect substantialconversion of calcium carbonate, CaCO3, into soluble calciumbicarbonate, CaHCO3,
 2. The method as claimed in claim 1 which includesthe step of (4) separating the precipitate containing color-impartingsubstances from the remainder of the liquid.
 2. pressurizing said liquidto greater than atmospheric in the presence of a fluid including anexcess of carbon dioxide, CO2, whereby said calcium carbonate, CaCO3, issubstantially converted to soluble calcium bicarbonate, CaHCO3, and 3.The method as claimed in claim 2 which includes the step of (5)directing the separated precipitate to a lime kiln for combustionconversion of CaCO3 to CaO.
 3. discharging the pressurized liquid toatmospheric pressure whereby said calcium bicarbonate, CaHCO3,substantially converts to insoluble calcium carbonate, CaCO3, as aprecipitate carrying therewith color-imparting substances present insaid paper mill effluent liquid.
 3. discharging the pressurized liquidcomprising essentially calcium bicarbonate, CaHCO3, to atmosphericpressure whereby said calcium bicarbonate, CaHCO3, essentially convertsto insoluble calcium carbonate, CaCO3, as a precipitate, carrying withsaid precipitate, in agglomerated fashion, color-imparting substancesand entities carried in said paper mill effluent,
 3. expose theresultant of steps (1) and (2) to a reduction in pressure to atmosphericpressure to convert soluble calcium bicarbonate to insoluble calciumcarbonate which forms a precipitate of said calcium carbonate, saidprecipitate attracting and carrying color bodies therewith, and 3.discharging the liquid comprising calcium bicarbonate into a settlingtank at atmospheric pressure whereby said calcium bicarbonate convertsto insoluble calcium carbonate as a solid precipitate which attracts andcarries color bodies in agglomeration or admixture therewith, 4.separating calcium carbonate precipitate and color bodies from saidliquid whereby said liquid is essentially decolorized and
 4. separatesaid precipitate and color bodies from said effluent.
 4. separatingeffluent liquid from precipitate and
 4. A method of reducing colorpresent in paper mill effluent liquid which comprises:
 5. In a system ofpulping wood by the kraft process wherein sodium hydroxide and sodiumsulfide are the active pulping chemicals combined with wood chips toattack the binding components and resulting in the liberation ofcellulose fibers and wherein black liquor recovered from the pulpingreaction is concentrated and burned to recover sodium values in the formof green liquor including sodium carbonate and sodium sulfide, whichgreen liquor is thence causticized with recovered lime to yield freshpulping chemicals and a by-product comprising calcium carbonate mud, aportion of said calcium carbonate being desirably burned to yieldreusable lime for said causticizing and resulting in by-product gasesrich in CO2, said pulping process and recovery process resulting in aconsiderable amount of mill effluent liquid which is normallyecologically undesirable due to discoloration caused by the presence ofcolor bodies; the improvement wherein said effluent mill liquid istreated in accordance with the following series of steps integrated intothe pulping process just described, said steps being as follows: 5.exposing said effluent to a stimulus to effect residual precipitation,formation of color bodies and stabilization of effluent pH to from 7.8to 8.4.
 5. returning said precipitated calcium carbonate to said limerecovery.
 6. A method of reducing color present in paper mill effluentliquid which comprises the following steps:
 7. The method as claimed inclaim 6, wherein said pressure measures about 20 to 100 pounds persquare inch gauge.